1. Field of the Invention
The invention is related to the field of telecommunications, and in particular, to communication systems and methods with phase encoding and phase shifting.
2. Description of the Prior Art
Communication systems use optical signals to communicate over optic fiber. Some of these systems transmit optical signals with a carrier frequency and sidebands. These sidebands carry the user data and are at frequencies slightly above and below the carrier frequency. In Optical Single Sideband (OSSB) transmission, one of the sidebands is almost completely removed.
One prior system for OSSB transmission uses electrical sub-carrier techniques. This system uses an electrical hybrid with a Dual-Electrode Mach-Zehnder modulator to generate OSSB signals. This prior system is essentially a replicate of the Hilbert architecture widely used in the RF domain. Because the electrical hybrid is a narrowband device, some prior systems first modulate the data onto a microwave sub-carrier and then pass the sub-carrier through the electrical hybrid. Although results of these systems show good sideband suppression, the non-linear transfer curve of the Dual-Electrode Mach-Zehnder modulator limits the modulation index and sub-carrier's frequency selection, especially when multiple sub-carriers are present. Another problem is the guardbands among microwave sub-channels impose limitations on the spectral efficiency. One of these systems also suppresses the carrier with a optical circulator and a Fabry-Perot filter. Suppression of the carrier is desirable because the carrier contains no useful information and occupies a large portion of the transmission power. Previous research into combining OSSB with carrier suppression is very limited due to previous complex architectures.
Prior electrical systems have used various encoding techniques for better binary signal performance. The simplest technique is non-return to zero (NRZ), where a binary 1 is represented by optical power within a bit period and a binary 0 is represented by zero optical power. Another technique is return to zero (RZ), where a binary 1 is an optical pulse while binary 0 means no optical power. Other types of encoding use phase transition of the signal to indicate a 1 or a 0, which is called phase encoding. One example of phase encoding is Manchester encoding. In Manchester encoding, a logical 0 is by a transition at the edge of a bit period, while a logical 0 is represented by a transition at the bit center. One problem with Manchester coding optical signals is the signal spectrum is doubled as compared with NRZ line coding.
Another example of phase encoding is Miller encoding, which is also called delay modulation. In Miller encoding, a logical 1 is represented with a phase transition at the bit center. A logical 0 is represented with no phase transition at the bit center. Two consecutive logical 0 has a phase transition at the boundary of the end of the first bit. Miller coding concentrates signal's power spectral density such that the signal's spectral occupancy is narrow. Miller coding has not been used in externally-modulated optical communication systems.